After stripping the interior and removing the back seats. There was a considerable pool of liquid under the seat behind the driver's.
A quick sniff test confirmed it was just water - but how did it get there?
A quick Google later and I was convinced it must be from the sunroof drain pipes. So I removed the side trim to reveal the black drain tubing - and what do you know, it was just flapping around not plumbed in to anything!
5 minutes later and I was thinking, what a quick fix for something that had me panicked to begin with.
I thought, I had better double check that it is no longer leaking - so I went ahead and slowly poured some water all over the roof. Somehow - there was still water getting in, but it wasn't from this tubing.
I was there for another half hour, pouring water, drying water until I finally found where it was coming from. It was coming from the rear hatch hinge! I really didn't want to remove the hatch to replace the hinge, so I just got some black sealant and squeezed it around the hinge then went to sleep and prayed.
The next day, I again poured water over the car and... there was no longer any ingress!